Add Mojo constant for "ash" and replace hardcoded strings

mash/common/config.cc

Index: mash/common/config.cc
diff --git a/mash/common/config.cc b/mash/common/config.cc
index f4e14f7809364732bcc8324771866f4fc8370ae5..3f6d3cc00be3681601bd3fcd7da3752b8e2a4481 100644
--- a/mash/common/config.cc
+++ b/mash/common/config.cc
@@ -4,6 +4,9 @@
 
 #include "mash/common/config.h"
 
+#if defined(USE_ASH)

[sky, 2017/02/24 18:40:07]

Conditional includes are after other includes.

[fwang, 2017/02/24 18:51:05]

Done.

+#include "ash/public/interfaces/constants.mojom.h"  // nogncheck
+#endif
 #include "base/command_line.h"
 
 namespace mash {
@@ -18,8 +21,13 @@ std::string GetWindowManagerServiceName() {
             kWindowManagerSwitch);
     return service_name;
   }
-  // TODO(beng): move this constant to a mojom file in //ash.
-  return "ash";
+#if defined(USE_ASH)
+  return ash::mojom::kServiceName;
+#else
+  LOG(FATAL)
+      << "You must specify a window manager e.g. --window-manager=simple_wm";
+  return std::string();
+#endif
 }
 
 }  // namespace common